The present invention relates to a method for calibrating sensors in a system comprising a plurality of sensors, which are arranged so that they can be subjected to the same load. In particular, the invention relates to calibration of a plurality of sensors of the same type, such as pressure sensors in a hydraulic system for a working machine.
Below, the invention will be described in connection with a working machine in the form of a wheel loader. This is a preferred, but by no means limiting application of the invention. The invention can for example also be used for other types of working machines (or work vehicles), such as a dumper, a backhoe loader, an excavator, or an agricultural machine such as a tractor.
In systems with many sensors, the relative values are often of interest, for example in case of drops in pressure or differences in temperature. However, the scattering between the sensors makes small differences difficult or impossible to measure, and it is difficult to achieve high measurement accuracy.
High measurement accuracy can be achieved by using sensors with narrow tolerances, alternatively by calibrating each sensor separately in production. Sensors with higher accuracy will result in a higher cost. Calibrating sensors separately in production takes time and will therefore also become expensive. Furthermore, the sensors drift over time, which is the reason why a continuous calibration is desirable.
It is desirable to achieve a method for calibrating sensors in a system which provides high accuracy during operation of the system, in a cost efficient way.
A method according to an aspect of the present invention comprises the steps of detecting the level of an operating parameter for a certain load level individually by means of at least two of said sensors, calculating a mean value of the operating parameter for said sensors for said load level, determining a deviation from the mean value for each of the sensors, and saving the value of the deviations for use in operation of the system.
Statistically, the calculated mean value has to be seen as the best value of a certain load level and is therefore used as a basis for the deviation (offset) at this load level. In this way, a higher accuracy can be obtained than the individual sensors can provide. The more sensors which are used for the mean value determination, the higher the accuracy becomes. A higher accuracy creates prerequisites for a better control during operation, which results in lower losses and thereby lower fuel consumption. Furthermore, prerequisites are created for a lower cost of the components in the system and a shorter lead time in production.
Preferably, the calibration is repeated continuously. Thereby, also a function check is obtained at each calibration.
A preferred application of the invention is a hydraulic system for operation of actuators (in the form of hydraulic cylinders) in a working machine for different functions. This system comprises, for example, pressure sensors for detecting a load pressure of each of the hydraulic cylinders and pressure sensors for detecting a pressure related to the output pump pressure.
Accordingly, in a hydraulic system where all, or a majority of the sensors can be pressurized with the same pressure, a mean value for all sensors which can be pressurized is calculated.
The hydraulic system is preferably load sensing. This means that the pump detects the pressure (a LS signal) from the activated hydraulic cylinders during operation of the system. The pressure signal then originates from said pressure sensors. The pump then sets a pressure which is a certain number of bar higher than the cylinder pressures. This brings about an oil flow out to the cylinders, the level of which depends upon how much the activated control valve is adjusted.
According to a preferred example, the method comprises the step of loading the sensors at a plurality of determined levels, saving the deviation values for each of the sensors for each of said load levels, and interpolating values for the deviation for the respective sensors within the range between said levels. In this way, a high accuracy can be obtained over the entire operating range in an efficient way. For example, linear interpolation between the different offset values can be utilized when calculating the useful sensor value.
According to another preferred example, the method comprises repeating the operating parameter detection several times and determining the mean value for each load as an average of the mean values from said repeated times. By means of performing the calibration continuously, possible drift over time can be handled. Furthermore, continuous calibration gives a higher quality (less individual scattering) of the system.
Further preferred embodiments of the invention and advantages associated therewith are apparent from the following description.